1. Field of the Invention
The present invention relates to telephone answering devices and sound signal manipulation, and more particularly to the means by which a voice signal may be better presented to an analog/digital converter in a digital telephone answering device.
2. Description of the Prior Art
Telephone answering devices (TADs) are well known and in common use throughout the United States. Recently, advancement in digital technology has influenced recent TAD design and production. Instead of using cassette tapes to record outgoing and incoming messages, TADs are now available that use digital memory storage as a recording medium. Digital memory storage, namely random access memory (RAM), requires the conversion of an analog voice signal generated by a microphone or phone line into a digital representation. This is not difficult, but does require a good clear signal for optimum performance.
Such good clear signals are easily generated by microphones attached to digital telephone answering devices (DTADs), but the same is not true for signals coming into the DTAD from the phone line. On present phone lines, voice signals from telephone lines have a dynamic range of about thirty or more decibels (dB). For this reason, some calls can be very clear while others can be faint or have much noise.
Standard analog to digital converters are eight bits "wide" and have a dynamic range of forty-eight decibels. Between the dynamic ranges of the phone line and the analog to digital converter is a difference of eighteen decibels. Thus, under worst case conditions, the effective dynamic range of the ADC would be limited to only eighteen dB. This would have severe voice quality consequences that make an eight bit ADC unusable. Conversely, if the dynamic range of the signal from the phone line was reduced to three dB, the ADC would have forty-five dB with which to work and eight bits would easily handle voice signals from the phone line.
Beyond the limitations of current eight bit ADCs with respect to phone line dynamic range, the use of sophisticated speech compression algorithms require clear signals for their operation. Without such clear signals, the use of speech compression in the storing and retrieving the digitized analog signal results in an overall degradation of the voice signal. The signal degradation is not experienced by clear signals, but only those that suffer a significant loss of integrity upon digitization, such as low level signals.
When an eight bit ADC attempts to convert a phone line signal from analog to digital representation, the signals that are close to the ADCs input range are fairly well converted. When the whole of the signals transmitted to a DTAD over the phone line are of low intensity, the digital signal created by the ADC will lose some of the information contained in the analog signal. As the analog signal from the phone line is of low intensity, only a small portion of the ADC's digitizing range is used. This leads to what is called "quantization noise" in the digitized signal and is noticeable when the digital signal is converted back into an analog signal upon playback. The quantization noise arises from the resolution available in an eight bit ADC. When the signal is low, the audio signal resolving power of an eight bit ADC is limited. In an extreme example of a faint signal, only four or five of the eight bit ADC's two hundred and fifty-six quantizing levels might be used for the whole of the signal.
When an analog signal is sampled by the ADC, the ADC changes the analog signal into the closest digital representation available, even though there is some disparity between the analog signal and its closest available digital representation. The more bits available to the ADC, the closer the digital representation will be to the analog signal. The loss of part of the analog signal experienced in digitization is called the quantization noise and can be modeled as the introduction of an unwanted signal when the analog signal is quantified by digitization. When the quantization noise of a digitized analog signal is high, the resulting analog signal created when the digital signal is played back has a tendency to become "granular" and reveal the discrete levels that were used to digitally record the original analog signal. When an analog signal dwells within a short range of digital levels, the quantization noise can be severe. Analog signals of low intensity with respect to the input range of the ADC have the tendency to dwell within such a short range.
One method that can be used to combat quantization noise is to increase the resolution of the ADC. By increasing the number of bits, an ADC has more levels available with which an analog signal can be represented. Where the number of bits can be increased, this is a successful means for reducing quantization noise. As a result, most DTADs demand that a twelve of thirteen bit ADC be used. The use of larger ADC's having more bits demands greater cost and more sensitive circuitry compared with eight bit systems. Additionally, many single chip microcontrollers used in DTADs have built-in ADCs and these ADC's are usually limited to only eight bits. If an eight bit ADC could be used, circuitry would be eliminated and costs could be reduced.
Another method is merely to use a fixed gain amplifier. In Millet, U.S. Pat. No. 4,794,638, issued on Dec. 27, 1988, a fixed gain amplifier is used. As the gain is fixed, no increase in the effective dynamic range is realized. High and low intensity signals are amplified by the same amount. Upon digitization, no improvement is seen in the loss of low intensity signals and quantization noise is not diminished.
In light of the decreased expense and design considerations, there is a need for a method by which eight bit ADC's may be used in DTADs with limited quantization noise for low intensity signals.